1. Field of the Invention
This invention relates to a liquid crystal composition, more particularly to a liquid crystal composition providing low rotational viscosity and high response speed.
2. Description of the Related Art
Liquid crystal displays presently in practical use can be classified into a twisted nematic (TN) liquid crystal display twisted by 90°, a super twisted nematic (STN) liquid crystal display twisted by 180° or more, and a liquid crystal display utilizing thin film transistors (TFT). It is well known that a single liquid crystal compound usually cannot satisfy a liquid crystal display to function appropriately. Therefore, a liquid crystal composition including different liquid crystal compounds is usually employed. In general, the liquid crystal composition is required to provide appropriate physical properties, e.g., refractive anisotropy (Δn), dielectric anisotropy (Δ∈), rotational viscosity (γ), and elastic constant ratio (K33/K11, K33 is bend elastic constant, K11 is splay elastic constant). For example, in a TFT display, the liquid crystal composition is required to provide a higher clearing point, a larger dielectric anisotropy, a larger refractive anisotropy, a low rotational viscosity, chemical stability, and a high voltage holding ratio (VHR). Lower rotational viscosity provides the display with a higher response speed.
In co-pending U.S. patent application Ser. No. 11/327,593, the applicant discloses a liquid crystal composition that includes:
10-90% by weight of one or more compounds of Type I having formula

1-40% by weight of one or more compounds of Type II having formula

1-50% by weight of one or more compounds of Type III having formula
and
1-40% by weight of one or more compounds of Type IV having formula

wherein in the formulas, rings A and B are independently phenylene or cyclohexylene; Z is a direct bond (no additional carbon atoms) or ethylene; n=1 or 2; m=0 or 1; and n+m>1; R and R′ are optionally substituted alkyl groups with 1-7 carbons in the chain, where the alkyl groups may be straight chain or branched, and one or more hydrogens can be replaced with one or more F or D atoms, and one or more carbon atoms can be replaced with —O; X and Y are selected from the group consisting of: F, H, Cl, and CF3 group.
Although the liquid crystal composition of the co-pending US patent application provides good physical properties, the rotational viscosity thereof still needs to be improved so as to achieve higher response speeds.